Toxoplasma gondii is an obligate intracellular parasite that is responsible for devasting disease in the developing fetus and in immunocomprised individuals. It has a complex life cycle involving a sexual stage in felines and an asexual cycle in any of a very large number of warm-blooded vertebrates. The asexual portion of the cycle involves an intraconversion between two forms, the fast-growing tachyzoite and the slow-growing, encysted bradyzote. The means by which T. gondii attaches to and invades virtually any cell in an appropriate host is largely unknown as are the triggers that cause it to differentiate from one stage to another. Understanding these complex processes is the ultimate aim of the work proposed in this application. This laboratory has focused on two proteins that have been implicated in attachment and invasion: the major surface antigens SAG1 (or p30) and the rhoptry protein ROP1 (or p60). In this competing renewal application, was used to accomplish four main goals: 1. Determine the function of SAG1 through analysis of parasites in which this gene has been specifically deleted or altered. 2. Identify the sequences and factors involved in the tachzoite-specific expression of SAG1. 3. Examine the function of ROP1 through targeted gene disruption. 4. Study the processing of ROP1 as it transits through the secretory apparatus. This latter goal involves determining the site of ROP1 cleavage, the compartment within the parasite where it occurs and the relationship between this processing and the targeting of ROP1 to the rhoptries.